One important use of lasers is in communications and information transfer, wherein one or more lasers transit modulated light intensity signals through one end of an optical fiber or cable. A receiver at the opposite end of the cable then converts the intensity variations in the laser light into useful information. Preferably, the intensity of the laser light is only a function of the modulation. Signal detection and separation of a plurality of light signals is easier if the laser light is free of noise. Moreover, more useful information can be transmitted if intensity variations can be spread over a wide frequency range or band. Ideally, the amplitude of any noise present in the laser's output is not a function of frequency.
A spectral analyzer is a device or instrument which displays the relative intensity of the frequency components of a laser light signal. Unwanted amplitude variations or changes in intensity represent noise. Laser noise is defined by the term Relative Intensity Noise (RIN), which is a measurement of optical power fluctuations over a specified band width. The observed spectrum from such an instrument represents the Relative Intensity Noise of a laser light signal. RIN is usually expressed in dB/Hz.
Semiconductor laser sources, such as laser diodes and laser diode arrays can be used alone or they can be used to pump another laser, such as laser using Nd:YAG (i.e., neodymium doped yttrium aluminum garnet as the lasant material. However used, a laser diode is activated or driven to produce laser light in response to the flow of DC electrical current from a power supply. When the RIN of the output of a diode-pumped solid-state laser is observed, it is characterized by distinct peak or a spectral region of high RIN. This peak is the result of the phenomenon of relaxation oscillation. A. E. Siegman, Lasers, University Science Books, Mill Valley, Cailf.; Chapter 25, 1986. Relaxation oscillations are small amplitude, quasi-sinusoidal exponentially damped oscillations. For diode pumped solid-state lasers (e.g., Nd:YAG), the spectral region is typically between 100 KHz and 1 MHz. Relaxation oscillation is thought to be dependent on the fluorescence lifetime of the lasant material, cavity lifetime of the laser system, and cavity losses.
One means of reducing RIN in a diode-pumped solid-state laser is to use electronic feedback. Kane, "Intensity Noise in Diode-Pumped Single-Frequency Nd:YAG Lasers and its Control by Electric Feedback", IEEE Photonics Technology Letters, Vol. 2, No. 4, April 1990. In particular, a photo-diode was used to sense the output of the laser system, and an amplifier was used to convert the output of the photo-diode into a phase shifted feedback signal which is added to the output of the DC Power Supply which is used to drive the system's laser diode. A positive phase shift, applied at all frequencies from the relaxation oscillation frequency up to the frequency where loop gain goes below unity, was needed to avoid instability. This phase lead was accomplished by designing the amplifier so that gain is rising as a function of frequency, as is the case of a differentiator.
The Kane feedback circuit does not take in consideration changes in the performance of the laser diode over the life of the laser diode. The relaxation oscillation frequency of a laser system changes over its life. Moreover, there is no teaching or suggestion as to how such a circuit should be arranged or physically located relative to the laser diode and its power supply to minimize noise and interference. Thus, there are many areas in which improvements can be made.